Heavy metal bearing wastewaters pose serious economic challenges to many large and small businesses in the developed world and especially in the third world, where they threaten the contamination of streams and soil. Heavy metals are highly toxic and tend to persist and concentrate when released into the environment. For this reason, even dilute streams of waste can pose a serious threat to life. The United States Environmental Protection Agency (EPA) has set discharge limits on heavy metal-bearing effluence. However, in some cases, the desired discharge limit is in conflict with the economic expense of metal ion removal and the level of current technology.
Current methods to remove metal from metal-bearing wastewater include processes such as reverse osmosis, chemical precipitation, carbon absorption, and ion exchange. These methods are effective in reducing the concentration of metal ions when present at high concentrations, but typically result in wastewater that still contains dilute concentrations of the metal ions. These dilute solutions are often above EPA discharge limits. Unfortunately, these methods are not efficient for removal of metal from large volumes of wastewater, nor wastewater containing low levels of metal contamination. In addition, these methods have high costs in terms of equipment as well as the energy requirement of these processes. These processes result in relatively large volumes of hazardous by-products which in turn must be disposed of properly, thereby further increasing the cost of decontamination of wastewater.
Polyalkyleneimines such as polyethyleneimine have been used to separate target metal from an aqueous solution (see U.S. Pat. No. 5,766,478). As described in U.S. Pat. No. 5,766,478, these synthetic polymers are highly branched, and it is thought that the branched structure not only gives these polymers a globular nature, but also a high charge density which was thought to allow for better binding with metal ions within the polymer. Furthermore, while it is thought that other polyamines, such as polyvinylamine could be used as the backbone of the polymer, that linear molecules would have an inherent drawback in that overfunctionalization could lead to insolubility in different pH ranges.
New technologies in wastewater treatment have involved use of microbial biomass such as algae, bacteria, fungi, and yeast. Biomass metal absorption has been conducted with both viable and dead or inactive cells. In these systems, the metal ions bind to the outside of the cells in the biomass and in the case of living biomass may also be internalized into the cells. In addition, microbial polysaccharides such as chitosan have been used for the removal of heavy metals or precious metal recovery from aqueous solutions (Hsien and Rorrer, 1995, Separation Science and Technology, 30:2455-2475).
Despite these technologies, the need exists for a high throughput continuous, cost effective method for removing metal ions from dilute aqueous waste streams, such that the effluent meets EPA allowable discharge levels.
The present invention is drawn to a method for separating at least one metal cation of interest from an aqueous solution. The method comprises processing the aqueous solution containing the metal cation of interest using a tangential flow filtration apparatus in the presence of a charged biopolymer. The aqueous solution is processed by adding it to the reservoir of a tangential flow filtration apparatus. The apparatus contains a polymer containing solution, wherein the polymer comprises a linear anionic molecule. The concentration of said metal ion in resulting filtrate is less than that originally present in the aqueous solution. The processing is terminating prior to breakthrough of the metal ion of interest by stopping the flow of aqueous solution into the filter apparatus.
The present invention is drawn to a method for separating at least a portion of a metal cation component from an aqueous component of an aqueous solution. The method comprises combining an aqueous solution that includes an aqueous component and a metal cation component with a polymer solution. The polymer solution comprises a linear anionic polymer component. At least a portion of said cation component binds to said linear anionic polymer component of said polymer solution. The combined solutions are directed across a membrane, wherein said aqueous component migrates through said membrane to form a filtrate, thereby separating at least a portion of said cationic component from the aqueous component.
The present invention is further drawn to a method for separating at least a portion of a metal cation component from an aqueous component of an aqueous solution. The method comprises combining an aqueous solution that includes an aqueous component and a metal cation component with a polymer solution, whereby at least a portion of said cationic component binds to a linear anionic polymer component of said polymer solution, wherein said linear anionic polymer component comprises xcex3-polyglutamic acid (PGA). The combined solutions are directed across a membrane, wherein said aqueous component migrates through said membrane to form a filtrate, thereby separating at least a portion of said cationic component from the aqueous component.
The present invention is further drawn to a method for separating at least a portion of a metal cation component from an aqueous component of an aqueous solution. The method comprises combining an aqueous solution that includes an aqueous component and a metal cation component, wherein the metal cation component is present at a concentration of about 10 ppm with a polymer solution, whereby at least a portion of said cation component binds to a linear anionic polymer component of said polymer solution. The linear anionic polymer component comprises xcex3-polyglutamic acid at a concentration of about 0.5% grams per liter. The pH of the combined solutions is maintained at in a range of between about 4 and about 5. The combined solutions are directed across a membrane, wherein said aqueous component migrates through said membrane to form a filtrate thereby separating at least a portion of said cationic component from the aqueous component. The combined solutions can then be treated such that at least a portion of said xcex3-polyglutamic acid-metal complexes are precipitated from the combined solution, forming a precipitate and a supernatant.
The present invention involves the use of microbially produced xcex3-polyglutamic acid (PGA) in conjunction with tangential flow filtration. The process, also termed polymer enhanced diafiltration (PEDF), uses a xcex3-linked amino acid polymer that is produced in high yield from the fermentation of a member of the Bacillus genus of bacteria, in particular Bacillus subtillus or Bacillus licheniformis. The process of the present invention retains and concentrates metal ions from dilute waste streams. In one embodiment, the concentration of the other components of the combined solutions are not altered, e.g., the PGA remains at constant concentration. In another embodiment, where the process is run until PGA-metal complexes precipitate in the retentate, soluble PGA is maintained at a given concentration by the addition of PGA containing solution to the process. The process uses filtration technology at high throughput and is scalable.
Furthermore, due to the insolubility of xcex3PGA-metal complexes at pHs of at least about 13, a further reduction in waste volume of at least about 10 fold is achieved through precipitation of the xcex3PGA by elevating the pH of the PGA-metal complex containing solution, e.g., the retentate or combined solutions (or combination thereof, if they are combined). The supernatant obtained typically the same concentration as the initial waste stream can be further processed according to the method of the present invention. The polymer used in the process is safe, non-toxic, naturally produced and biodegradable. The polymer can be regenerated and the metal recovered using techniques such as solubilization followed by precipitation of the metal ion or techniques such as electroplating. In addition, if it is not necessary to regenerate the polymer, the precipitated polymer can be dried prior to disposal. The precipitate, comprising xcex3PGA-metal complexes can also be treated to release the bound metal or, because the xcex3PGA is non-toxic, the precipitate can be burned and the metal oxides recovered.
The process of the present invention results in at least about an 80-fold decrease in metal containing waste volume during the filtration step and an additional decrease of at least about 10 fold in the precipitation step. Resulting in an overall concentration of about 800 fold. In addition, the filtration process results in the retention of copper, lead or cadmium of at least 99%.
As described herein, a linear, microbially produced polymer, xcex3PGA, is highly effective at removing metal ions from dilute aqueous solutions. Despite the expectation that a linear molecule would not function as well as a branched molecule, as demonstrated herein, a tangential flow filtration process using xcex3PGA is able to remove copper, lead and cadmium ions to less than 1.3, 0.0015 and 0.005 ppm respectively. The metal binding capacity of PGA is extremely high. For example, the capacity of PGA for copper was greater than 180 mg/g of dry PGA, which is produced via a high yield fermentation process that results in recovered yields of greater than 50 g/l. The high binding capacity and high yield production are important factors in reducing the cost of removing metals from dilute aqueous solutions.